1. Field of the Invention
This invention relates generally to an electric drill or drilling machine, and particularly to an electric drilling machine equipped with an electric control system.
2. Description of the Prior Art
Electric drilling machines include a drive motor for rotating a drill bit. In general, when the drilling process is completed, an electric current passing through the drive motor drops to a no-load level.
Japanese published unexamined utility model application No. 57-96719 corresponding to Japanese published examined utility model application 61-37450 discloses a processing stroke control system for a drill unit. This system includes a meter-type relay which senses a no-load electric current through a drive motor to detect the completion of the drilling process. The detection of the completion of the drilling process is used in the processing stroke control.
The no-load current through the drive motor depends on various factors, such as the voltage across the drive motor, the ambient temperature, and the mechanical loss. Furthermore, in the case where the rotational speed of the drive motor is adjusted in accordance with the diameter of the drill bit, the no-load current through the drive motor varies with the diameter of the drill bit. The Japanese utility model application 57-96719 fails to consider such variations in the no-load current through the drive motor.